Hitherto, hollow stiffening ribs of the kind under discussion have been made by applying bulky, low strength, ropes of paper, wooden slats or other cheap former elements, to one side of a sheet-like article or article portion and subsequently covering those elements with a layer of the settable rib material which sets and becomes bonded to the main body of the sheet intermediate the elements, but is held away from the main body by the elements so constituting filled or hollow ribs.
While the above described method of providing stiffening ribs is generally satisfactory, it is not entirely so, primarily because of the difficulty of holding the former elements in position relative to each other, especially if it is desired to run stiffening ribs in two directions substantially at right angles to each other. These other methods substantially increase the weight and/or cost of the product.
One method of overcoming these problems has been to use a former comprising a foraminous sheet in which the areas between adjacent holes comprise a grid of spaced apart, arch-sectioned longitudinal members and spaced-apart, arch-sectioned transverse members and said members being integrally united at the respective intersection points.
Though this method provides a great improvement over previous arrangements it still has several disadvantages. In order to produce the arch-sectioned members used in this known method the material used to make the former must be substantially rigid and have sufficient mechanical strength to support a layer of settable plastics material. A distinct disadvantage of known formers, when applied to the production of shaped articles having complex curves, is their lack of flexibility and inability to follow the complex curves without losing or distorting the original designed shape and losing their effectiveness.